Seabirds are prominent components of the North Atlantic marine
environment, and their parasites offer an insight into seabird ecologic
interactions. Parasites also provide vital information on historic
biogeography of host associations and thus may reveal broad changes in
the marine ecosystem. Helminths of common murres (Uria aalge) and
thick-billed murres (Uria lomvia) in the northwest Atlantic marine
environment were assessed to determine parasite community composition
and changes in their parasite fauna since the 1960s. In total, 623
helminths, which represented Digenea, Eucestoda, Nematoda, and
Acanthocephala, were recorded from 100 common and thick-billed murres
collected from breeding colonies along the coasts of Labrador,
Newfoundland, and Greenland. Parasite communities differed from those
reported from the 1960s, and more than 85% of the specimens were
tapeworms (mostly in the genus Alcataenia). The high prevalence (26%)
and mean intensity (14.6) of Alcataenia longicervica, a Pacific species
recorded recently from Newfoundland, indicated that this tapeworm was
established in the Atlantic by 2006. Significantly higher A longicervica
prevalence (>53%) and mean intensity (27.3) in the murres from
Greenland and in wintering murres compared with murres from breeding
colonies in Labrador and Newfoundland suggest a mechanism for the
introduction of this species to the Atlantic. Periodic mixing of
populations of Thysanoessa species, the euphausiid intermediate host of
Alcataenia, occurs along the seas adjacent to the North Pacific and
those along the Siberian Arctic. The mixing of infected Thysanoessa
likely exposed North Atlantic and Arctic murres, which are
geographically isolated from Pacific murres, to this tapeworm. The
greater prevalence of A longicervica in thick-billed murres was
consistent with diet analyses, which revealed a greater proportion of
euphausiids.